The conventional approach to commercial sterilization of food is thermal processing using slow heating retorts with slow cooling in cans afterwards. This process is highly effective for inactivating microorganisms, enzymes and microbial spores that cause food spoilage in shelf stable foods. However, due to the lengthy thermal process caused by slow heat penetration to the can's center and subsequent slow cooling, the prolonged thermal process significantly softens texture in vegetables and meat and induces an undesirable change in flavors. Pasta and rice lose their chewiness and become mushy; meat also loses its chewiness and becomes soft in texture; vegetables also lose their crunchiness and become soft. The color in dairy products takes on a scorched, brown color while vegetables fade in color. Delicate products such as those containing cheese and dairy products such as macaroni and cheese can not be made using conventional retorting because they caramelize, take on a very undesirable flavor and become inedible. Even faster heating systems such as aseptic heating suffers from a prolonged heating up time and holding period (to achieve commercial sterility) which causes scorched, undesirable flavors along with a prolonged cooling period.
A recently developed alternative to conventional thermal processing is the use of ultra-high pressure to destroy spoilage-causing microorganisms and spoilage-related endogenous enzymes. The use of ultra-high pressure for food processing has been made possible by recent advances in the engineering of devices capable of delivering the necessary pressures to commercially useful amounts of foods. Machinery capable of industrial high pressure food sterilization is available, for example, from Flow International Corp. (Kent, Wash.), Mitsubishi Heavy Industries (Tokyo, Japan), Kobe Steel (Kobe, Japan), ABB Autoclave Systems, Inc. (Vasteras, Sweden), and Engineered Pressure Systems, Inc. (Andover, Mass.).
It has been suggested that low acid foods can be sterilized using a combination of ultra-high pressure and high temperatures. International PCT Application No. W097/21361 discloses a process for heating low acid canned foods to a temperature of 80-99.degree. C., then pressurizing them to a pressure of 50,000-150,000 psi, followed by decompression and transfer to a cooling trough. The process takes advantage of the adiabatic temperature rise induced in the food upon pressurization, of approximately 20.degree. C. at a pressurization of 100,000 psi, and the corresponding adiabatic temperature decrease upon depressurization, to reduce preheating and post-chilling times. However, the process disclosed in this reference in fact does not achieve sterility at commercially useful pressurization times. For example, a disclosed pressurization cycle of 90,000 psi at 85.degree. C. for 30 minutes provides an insufficient lethality, i.e., was insufficient to obtain a reduction of 1012 in the population of botulism spores (a 12D reduction). An alternate pressure sequence disclosed in this reference is 90,000 psi at 98.degree. C. for five minutes. This provides a lethality that computes to be in excess of the standard 12D requirement, yet has been found by the present inventor to result in an insufficient sterilization of B. cereus, which tend to multiply to unacceptable levels within one week of pressurization. The only sequence disclosed that would achieve true sterility in this reference is 90,000 psi at 98.degree. C. for 30 minutes, which is too long to be of commercial utility, and which would result in a highly overcooked food product.
Other researchers have suggested that exposing foods to pulses of ultra-high pressurize effective at reducing microbes for increased refrigerated microbial stability. Torres, J. A. and Aleman, G., Pressure Pulsing: Improving UHP Effectiveness, OSU Food Process Engineering and Flow International Corp. However, these researchers have not suggested the use of ultra-high pressure pulsing to achieve commercial sterility.
Still other researchers have found that a combination of heat and ultra-high pressurization is not effective at achieving commercial sterility, due to the survival of bacterial spores. Mallidis, C. G. and Drizou, D., Effective Simultaneous Application of Heat and Pressure on the Survival of Bacterial Spores, Journal of Applied Bacteriology, 71, p. 285-88 (1991). This reference teaches that spores are extremely heterogeneous in their sensitivity to heat and pressure, and concludes that a combination of pressure and heat is not effective for the preservation of liquid foods.
There thus has not yet been developed a commercially viable method of commercially sterilizing low acid foods while preserving the flavor and texture benefits of short-exposure to ultra-high pressurization.